Methods for quantitative studies of seafloor hydrothermal systems using 3D visual reconstructions

3D visual mapping of the seafloor has found applications ranging from environment monitoring and survey of marine minerals to underwater archaeology and inspection of modern artificial structures. However, the attenuation of light is significantly more pronounced in water than in air or in space, and so in order to obtain underwater images in colour, it is typically necessary to be within 2–3 m of the seafloor. In addition to the high risk of collision when operating underwater vehicles at such low altitudes, the limited area of the seafloor covered in each image means large area surveys require a significant investment of time. In this research, we aim to increase the efficiency of mapping large areas of the seafloor by developing an underwater imaging system that can take colour images at ranges of more than 10 m, so that each image can cover a larger area, together with the necessary algorithms to automatically process the data it obtains. The system was deployed to map artificial hydrothermal vents in Iheya North field using the ROV Hyper-Dolphin in October 2012. The surveyed area is of particular interest to the research community, as multiple artificial vent holes were drilled during a mission in 2010, which locally impacted the flow of hydrothermal fluids. In this paper, we describe the methods used to process the data that the imaging system obtains and demonstrate how the mapping data can be used in quantitative studies of the seafloor. Habitats of Shinkaia crosnieri squat lobsters, which are abundant in the hydrothermally active areas, are identified in the maps and their population density calculated, and the amount of hydrothermal deposits that have grown on the artificial vent is derived from the mapping data. The work demonstrates how 3D visual mapping can be applied to benthic biology and geological studies.